By George Melchior
By now, we’re all well into our preseason preparations for the upcoming winter: Equipment checks, material orders, site walks, contract updates and renewals. And, while the Farmers’ Almanac is predicting a cold, snowy winter across the northern U.S., there’s no doubt we’re all knocking the dust off of our snow management equipment and figuring out our plow, sand, salt and removal logistics. These are busy times, which is why I feel compelled to remind you to remember the ice. When the weather channels become fixated on snowfall; when the ski industry is praising their big rebound from last season; when friends and neighbors drone on about their woes of endless blowing and shoveling of walks and driveways, remember the cardinal rule of wintertime premises liability: Ice is dangerous.
Watch for warning signs
For ice to form, the temperature must be at or below freezing, and moisture must be present. When we talk about temperatures with regard to ice formation on a surface, we’re talking about surface temperatures. Temperature is simply the measure of heat present in a substance or object. In winter, where heat is limited, surface temperatures can be quite dynamic and change as a result of the movement or transfer of this limited heat. The three modes of heat transfer that influence surface temperatures are conduction, convection and radiation. Each affects surface temperature in different ways.
Moisture can come from fog or heavier precipitation like sleet, snow and rain. Moisture can also migrate to surfaces from melt-refreeze of snow stockpiles, vehicle tires, overflow of clogged storm water systems, etc. Additionally, moisture can be persistent. It can collect and stay in puddles, potholes and other low points of streets and parking lots for long periods as a result of slow evaporation and/or inadequate drainage.
However, both temperature change and moisture accumulation are functions of time, which makes both components of ice formation predictable. With an understanding of the factors that influence heat transfer and moisture accumulation, ice managers can predict when surface temperatures will fall below freezing, and when moisture will be present. Here’s what to watch for:
Air temperature. As air temperatures trend below freezing for a few weeks, the earth will fall below freezing (usually by mid-December), and will stay below freezing for the remainder of winter. From then on, we’re watching air temperatures to predict when surface temperatures will rise above freezing, and when they’ll fall back below freezing. Remember, surface temperatures do not instantaneously match air temperatures. Just because the air temperature for a day in January is in the mid-40s, that doesn’t mean the surface temperature is above freezing. The change in surface temperature takes time.
A good rule of thumb to start with is four hours at 40˚F. That is, on conduction only (no wind, no sunlight), it can take an asphalt parking lot surface up to four hours to rise above freezing when the air temperature is 40˚F. If the air temperature is higher, then the parking lot will warm faster, and if the air temperature is cooler, then the lot will warm slower.
Low pressure systems. Low pressure systems tend to result in precipitation. That is because the general airflow for a low pressure system is inward, toward the center of low pressure. When that happens, the air is pushed up vertically. As that vertical air rises, it cools, which can cause water vapor to condense in the form of clouds, rain, sleet and/or snow. A rapid fall in barometric pressure is generally indicative of an approaching storm.
A good rule of thumb for anti-icing is if the barometric pressure is at or below 30.20 inches of mercury (in-Hg) and is dropping at a rate of more than 0.20 in-Hg/per hour, then you can expect clouds, wind and/or precipitation inside of 12 hours (adjust a bit lower for areas more than 4,000 feet above sea level).
Dew point. The temperature at which water vapor in the air will condense and form moisture is called dew point. The dew point is a relative measure and can never be higher than the air temperature. However, if the dew point temperature converges with the air temperature, then the moisture in the air will condense and form fog, rain and/or snow.
For anti-icing efforts, a good rule of thumb is that a dew point spread (difference between dew point and air temperature) of 2˚F or less will result in precipitation. If the dew point spread is trending toward convergence with the air temperature and is approaching a spread of 2˚F, and the surface temperatures are near freezing, anti-icing measures should be taken.
Cloud cover. Clear skies during the day provide an opportunity for the sun to warm surfaces via radiation. Radiation is the fastest mode of heat transfer to and from a surface. As such, a clear sky can be a double-edged sword.
While it allows for exposed surfaces to be warmed quickly by the sun, it also allows for melting of stockpiled and perimeter snow. When the sun goes down, those same warm surfaces will reverse-radiate their heat back into the open atmosphere, causing surfaces to rapidly equilibrate with sub-freezing ground and air temperatures. The melted snow may rapidly refreeze on walkways and parking lot surfaces. If the sun sets between 4 and 4:30 p.m., and air temperatures and ground temperatures are below freezing, walking surfaces can fall below freezing in less than an hour placing patrons at risk of injury on inconspicuous patches of ice.
The best way to combat the ice formation from reverse-radiation is to remain tidy with snow stockpiles, and make it a point to get anti-icing chemical down before sunset. If clear skies are to persist throughout the night, those anti-icing measures will prove effective the next morning as well.
Best of luck in your preseason preparations. Check your equipment, inventory your PPE, order your material and get those contracts buttoned up. But, also remember winter’s true danger: ice. Managing the snow is important, but managing the ice saves lives and will save you from costly personal injury claims.
George Melchior, ASM, is a registered architect and professional engineer and owns GVM Consulting, based in Portsmouth, NH. Contact him at firstname.lastname@example.org.